U.S. Pat. No. 7,149,148B2 and EP0199571A1 disclose detection apparatuses which process an echo signal by using a dual demodulation or dual sweep demodulation. The dual demodulation is one type of demodulating methods in which two systems of local signals having frequencies different from a transmission signal are prepared, the two local signals are each multiplied by a reception signal and the results of the two multiplications are added to generate a beat signal. A detection of a target object is performed based on the beat signal generated by the dual demodulation.
The present inventors found out a problem that side lobes (so-called range side lobes) of a peak waveform of a target object become large when the dual demodulation described above is applied to the detection apparatuses. If such range side lobes are generated, the target object to be detected is buried in the range side lobes. Therefore, it is impossible to detect the target object.